1. Field of the Invention
This invention relates to a process for the hardening treatment of sintered members which can harden all or part of an iron-based sintered member without producing any heat distortion, and can thereby improve its wear resistance.
2. Description of the Related Art
Conventionally, synchronizing hubs which are iron-based sintered parts are frequently used in the transmissions of four-wheeled motor vehicles. Since these synchronizing hubs come into face contact with counter parts, they are required to have high wear resistance. In the case of such parts requiring wear resistance, it has been conventional practice to subject sintered parts to a thermal refining treatment comprising quenching and tempering, induction hardening, impregnation treatment (also known as an infiltration treatment) or the like and thereby improve the hardness and wear resistance thereof.
However, the above-described conventional hardening processes have the following problems.
1) Since the thermal refining treatment is carried out by heating a whole sintered member and cooling it rapidly, the sintered member may undergo a heat distortion. Consequently, it has been difficult to obtain high dimensional accuracy stably.
2) Induction hardening has the advantage that the sintered member retains high dimensional accuracy. However, the induction hardening equipment is expensive. Moreover, separate hardening coils are required for different types of sintered members, so that treating costs may be increased because of limited flexibility. Thus, induction hardening has the disadvantage that the overall cost is high.
3) The impregnation treatment is a method in which another infiltration metal is melted and infiltrated into a sintered member to fill the pores of the sintered member and thereby improve the strength and toughness thereof. To this end, powdered copper is usually used as the infiltration metal. However, it has been impossible to achieve sufficient wear resistance.
4) Another process is to harden a sintered member by modifying the composition of a sinterable powder mixture or allying it. However, since this process involves tempering or cementation hardening, a heat distortion may be produced. Consequently, it has been difficult to obtain high dimensional accuracy stably.
An object of the present invention is to solve the above-described problems by providing a process for the hardening treatment of sintered members which can impart sufficient wear resistance thereto at low cost without causing a reduction in dimensional accuracy due to heat distortion.
In order to accomplish the above object, the present invention provides a process for the hardening treatment of sintered members which comprises the steps of compacting an iron-based sinterable powder mixture to form a compact; coating the compact with a coating material comprising a Nixe2x80x94P alloy powder containing 7 to 13% by weight of phosphorus so as to give a coating weight of at least 0.1 mg/mm2; sintering the compact coated with the Nixe2x80x94P alloy powder to produce an iron-based sintered member; and hardening the iron-based sintered member by cooling it.
In the above-described hardening treatment process, the coating material applied to the compact comprises a powder, so that the coating material can be easily applied and used for purposes of local treatment. Moreover, since the aforesaid coating material comprises a Ni alloy, the heat of sintering causes the coating material to penetrate into the compact and diffuse into its iron matrix to form a solid solution, and the subsequent cooling step can transform the iron matrix into a martensitic structure and thereby form a hardened layer. This martensitic structure is a very hard metallographic structure and can hence improve the wear resistance of the sintered member. Moreover, since the coating material contains 7 to 13% by weight of P, this P penetrates and diffuses deeper into the compact, so that a hard steadite structure (Fe3P) is precipitated to form a hardened layer and thereby improve the wear resistance of the sintered member. The thickness of the hardened layer can be controlled by varying the coating weight of the coating material.
In the process for the hardening treatment of sintered members in accordance with another embodiment of the present invention, the aforesaid coating material is applied so as to give a coating weight of 0.1 to 1.2 mg/mm2, the aforesaid sintering is carried out at a temperature in the range of 1,000 to 1,300xc2x0 C., and the aforesaid cooling is carried out at a cooling rate of not less than 10xc2x0 C./min. until a temperature of 200xc2x0 C. is reached.
Since the aforesaid sintering temperature is in the range of 1,000 to 1,300xc2x0 C., the compact can be treated in a conventional sintering furnace without requiring special equipment or high costs.
The process for the hardening treatment of sintered members in accordance with the present invention can impart high wear resistance thereto at low cost without producing any heat distortion.